The known state of the art includes numerous kinds of electrochemical measuring systems or also measuring probes which have at least one electrochemical second order half-cell, for example ion-sensitive, potentiometric or redox measuring probes. These kinds of measuring probes are in everyday use for electrochemical measurements in analytical laboratories and process systems.
An electrochemical second order half-cell has a metallic conductor element arranged in an electrolyte which includes a saturated solution of a low-solubility salt of the metal of the conductor element. The conductor element is preferably coated with a surface layer of a low-solubility salt of the same metal of which the conductor element itself consists, wherein the salt contained in the surface coating and the salt dissolved in the electrolyte normally have the same anion. As is well known, the electrical potential of the conductor element, i.e. the electromotoric force E of this kind of a half-cell, is defined by the Nernst equation
                              E          =                                    E              ⁢                                                          ⁢              °                        -                                                                                R                    ⁢                                                                                  ⁢                    T                                                        z                    ⁢                                                                                  ⁢                    F                                                  ·                ln                            ⁢                                                          ⁢              a                                      ,                            [        1        ]            wherein E° represents the standard electrode potential, R stands for the universal gas constant, T for the absolute temperature, z for the valence of the ion that determines the potential, or for the valance change in the case of a redox reaction, F stands for Faraday's constant, and a stands for the activity of the ion that determines the potential.
Half-cells of the known state of the art are based for example on the combinations silver/silver chloride (Ag/AgCl), calomel (Hg/Hg2Cl2), mercury sulfate (Hg/HgSO4) or thalamide (Hg(Tl)/TlCl). While the redox combination of metal and metal ion is basically the determining factor for the potential, the respective metal ion activity in electrochemical second order half-cells is defined by the solubility product of the low-solubility metal salt and indirectly by the activity of its anion.
Measuring electrodes often consist of at least one reference half-cell and at least one measuring half-cell, with the latter also being referred to as glass half-cell, as for example in the case of a conventional pH electrode. The difference in the potentials occurring between two half-cells can be measured as a voltage. This measured voltage represents a measure for the ion activity or ion concentration. In particular for reference half-cells or for reference electrodes, several monitoring methods are already known.
A reference electrode for potentiometric measurements with a liquid electrolyte as well as a method of monitoring the fill level of the electrolyte by means of resistance measurements between several conductor elements, at least one of which is not immersed in the electrolyte, is disclosed in commonly-owned and co-pending US patent publication 2006/0070889 A1, by the present inventor.
In US patent publication 2005/0040038 A1 to Berger, a reference electrode with two separate chambers is disclosed wherein each chamber has a filling of electrolyte with a conductor element immersed in it. Since only one of the chambers is in contact with the measuring medium through a so-called liquid junction, for example a diaphragm, it is possible to detect changes taking place in the chamber that is in contact with the measuring medium by making a comparison measurement with the other chamber.
In U.S. Pat. No. 6,685,807, issued on 3 Feb. 2004 to Meier and commonly-owned by the assignee of this application, a method of monitoring the aging of a potentiometric measurement probe is disclosed, wherein a secondary conductor element is arranged at a shorter distance than a primary conductor element from the end of the measuring probe that is immersed in the measuring medium. An advancing electrolyte impoverishment first affects the measurement data of the secondary conductor element and is thus indicative of a progressing deficiency. Furthermore, a method for determining the remaining operating life of a potentiometric measuring probe is disclosed in published application DE 101 00 239 A1, where a secondary conductor element is arranged nearer to the end of the measuring probe that is immersed in the measuring medium than a primary conductor element. The remaining operating life is determined based on the difference between the respective potentials of the conductor elements as well as the basic operating time that has already elapsed.
The methods that are known from the prior art are oriented primarily towards monitoring the properties of the electrolyte and are based on the assumption that the conductor elements are always functioning in perfect order. However, this cannot simply be taken for granted, in particular for second order half-cells which are exposed to temperature fluctuations.
A known disadvantage of the electrochemical second order half cell lies in the considerable, non-uniform and partially discontinuous temperature dependency of the potential E of the conductor element. The reason for the not very transparent temperature dependency of these kinds of half-cells lies primarily in the generally strong temperature dependency of the solubility products of low-solubility compounds which causes the half-cell potential E to be temperature-dependent. The situation is further aggravated by the use of saturated saline solutions with the anion determining the potential, for example saturated solutions of potassium chloride or potassium sulfate in the electrolyte. The superposition of the two effects on each other leads to a highly undesirable discontinuous temperature behavior of the half-cell, as the solubilities of these salts likewise exhibit a pronounced temperature dependency. As an additional problem, the Nernst slope, i.e. the factor preceding the logarithmic term in equation [1], is likewise temperature-dependent. This is for example the reason why, if a reference electrode with a half-cell of this kind is used in setting up a measurement chain with a second measuring- or indicator half-cell, it is never possible to obtain an exactly defined point of intersection of isothermal curves as required for example for a temperature correction according to DIN 19265.
In addition to the temperature dependency that has just been described, measuring probes with second order half-cells suffer from two further severe drawbacks. It is possible in case of temperature changes that parts of the salt forming the surface coating get dissolved in the electrolyte and will be precipitated from the electrolyte when the temperature falls again. This manifests itself first of all in a pronounced hysteresis behavior, i.e. in a deviation of the electric potential when the temperature returns to the lower level that existed at the beginning.
In addition, given that in all of the aforementioned reference half elements the actual measuring ion activity is set by way of the solubility product of a low-solubility salt, and since in heterogeneous equilibriums of this kind between the solution and the solid substance there are certain kinetic retardants inherent which present themselves as over-saturation, the setting of the potential in these reference electrodes after a temperature change occurs only after a certain time delay. This leads to a sluggish response behavior in potentiometric measurements, which can be very harmful in industrial applications.
Frequent temperature changes can even lead to a complete decomposition or dissolution of the metal salt coating from the metallic conductor element, causing a deterioration of the response behavior of the half-cell and/or even an irreversible destruction of the half-cell.
In measuring probes that are installed in a process system, it would be desirable to have the capability to monitor the proper functioning of individual measuring probes and in particular the proper functioning of their half-cells, in order to be able to identify defective measuring probes quickly, simply and reliably and to replace them at the right time.
It is therefore the objective of the present invention to develop a method of monitoring an electrochemical second order half-cell, in particular to monitor the ability of the conductor element to function correctly, as well as to provide a measuring probe that is suitable to perform the method.